State formation is the process of the development of a centralized government structure in a situation where one did not exist prior to its development. State formation has been a study of many disciplines of the social sciences for a number of years, so much so that Jonathan Haas writes that "One of the favorite pastimes of social scientists over the course of the past century has been to theorize about the evolution of the world's great civilizations." The study of state formation is divided generally into either the study of early states (those that developed in stateless societies) or the study of modern states (particularly of the form that developed in Europe in the 17th century and spread around the world). A number of different theories explain the development of early states and modern states, and many of the academic debates remain prominent in different fields of study.